The pump sucks the air in the upper part of the tube, thus the pressure in the upper part gets down from the atmospheric pressure at the surface of the mercury in the cup. So the mercury in the tube goes up towards the lower pressure. The height of the mercury column in the tube balances the pressure in the tube at the level of the free surface in the cup. The maximum suction in the upper part of the tube will be zero pressure i.e. vacuum. Thus the maximum pressure difference in the tube will be equal to one atmospheric pressure which will be balanced by 76 cm of the mercury column in the tube. Above the height of 76 cm the mercury will not rise and hence it will not reach the pump because the tube is 1 m = 100 cm long.
